Random physics question regarding light

[Brother Oni]

Please excuse any mis-understandings I have about light - I'm a biochemist, not a physicist.

As I understand it, an object is visible because it reflects light, or rather, it absorbs light then emits it at a visible wavelength dependent on the material in question when the excited electrons return to their resting state.

This emitted light is then received by your eye, which your brain interpets.

So what happens if a theoretical substance absorbs all light that hits it? It's not invisible as light can't pass through it, but also you can't see it directly as it doesn't emit any light for you to see.

Would it appear just as a shadowy material or just a eye-aching blackness? Or something else entirely?

[Eloel]

That's what black is, actually, as far as I know.

[Tarinaky]

What you describe is called, in layman's terms... 'black'.

[Ranger Mattos]

As far as I know, if a substance reflected absolutely no light, it would be effectively invisible, but would still cast a shadow. I'm probably wrong though.

[Seonor]

An object that doesn't reflect light is simply black to the eye. But nothing absorbs 100% of all light, it only seems that way because the human eye can't detect small rest of the light that is still reflected.

There is some research to create a "perfect black" which would absorb absolutely every light but until now there is no such thing, even if there are some promising developments in that direction. It would still look like normal black to human eyes.

[Eloel]

As far as I know, if a substance reflected absolutely no light, it would be effectively invisible, but would still cast a shadow. I'm probably wrong though.

For something to be invisible, light from the other side needs to come through it.

[The Rose Dragon]

Well, you probably know that your eyes don't exactly see much - they can't even interpret the world as three-dimensional. That's what the brain does. Therefore, even if your eyes don't receive any visible light from an object, the brain will interpret the absence of light in an area of the retina as a black object. It should not ache your eyes any more than sensory deprivation tanks, though suddenly replacing the object with a light source after a while can hurt the eyes. However, if it is completely absorbent, you should not be able to see texture or shape except by contrast.

[Eldan]

As far as I know, if a substance reflected absolutely no light, it would be effectively invisible, but would still cast a shadow. I'm probably wrong though.

Not invisible. Just absolute black. Which would really stand out against almost anything.

Normal black objects you see still have textures, reflections, dust particles and so on, so that gives them some shape. A perfectly black object would basically just be a black shape.

[Form]

Relevant link if you want more detail:

http://en.wikipedia.org/wiki/Blackbody

[ZombyWoof]

Would it appear just as a shadowy material or just a eye-aching blackness? Or something else entirely?

Close your eyes in a dark room.

There you go.

There's no functional difference between something that absorbs all visible light and something that reflects some/all of it when there is none to reflect, so just create an environment where there's not enough light for your eyes to detect and voila.

[Asta Kask]

Doesn't a black hole do this? Sure there's Hawking radiation, but if you take a really supermassive black hole then that's negligible.

[shawnhcorey]

So what happens if a theoretical substance absorbs all light that hits it?

It would be black. That's why black holes are called black. They adsorb almost all light. They to emit light as Hawking radiation.

Also, since heat is light at a low frequency, it's temperature would be absolute zero. For comparison, the background radiation of the universe is about 3K or -270C or --454F. Absolute zero is 0K or -273C or -459F.

[valadil]

Doesn't a black hole do this? Sure there's Hawking radiation, but if you take a really supermassive black hole then that's negligible.

Effectively, yes. It's not absorbing the light in the same way a blue object absorbs all red and green. But its gravity is such that light can't get away.

A fully absorbent sphere would be invisible. It can't be seen. It would not be transparent, so it would still be detectable by vision even if you're seeing the absence of the object rather than the object.

[grimbold]

there are things called metamaterials that can actually refract light around themselves rendering them essentially invisible

[Ranger Mattos]

For something to be invisible, light from the other side needs to come through it.

Not invisible. Just absolute black. Which would really stand out against almost anything.

Normal black objects you see still have textures, reflections, dust particles and so on, so that gives them some shape. A perfectly black object would basically just be a black shape.

Note to self: Don't attempt to do physics late at night. I knew I was wrong this morning before math, but didn't have any opportunity to get on and correct myself until now.

[Zexion]

Well, perfect black would look absolutely, completely black, and any three-dimensional perfect black object would look like a silhouette, I believe.

[thubby]

a perfectly black object would be difficult to even interact with. you could never know how close you are to the nearest face.

[golentan]

It would look like my soul (read, as everyone else has sad, black outline filled with black without texture or detail to mitigate the all pervasive blackness).

Thubby: I think you could still get a fairly good idea comparing outlines with parallax. Though I'm not sure how close you'd need to get to get an accurate fix on it. It... might be pretty close.

Edit: Also, how hot would this object get leaving it in the sun. Ouch!

[Ted_Stryker]

Relevant link if you want more detail:

http://en.wikipedia.org/wiki/Blackbody

Quoting an excerpt for emphasis:

A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum that depends on the body's temperature. At Earth-ambient temperatures this emission is in the infrared region of the electromagnetic spectrum and is not visible. The object appears black, since it does not reflect or emit any visible light.

The thermal radiation from a black body is energy converted electrodynamically from the body's pool of internal thermal energy at any temperature greater than absolute zero. It is called blackbody radiation and has a frequency distribution with a characteristic frequency of maximum radiative power that shifts to higher frequencies with increasing temperature. As the temperature increases past a few hundred degrees Celsius, black bodies start to emit visible wavelengths, appearing red, orange, yellow, white, and blue with increasing temperature. When an object is visually white, it is emitting a substantial fraction as ultraviolet radiation.

In fact, what would happen with a perfect absorber is that it would change temperature until the amount of radiation it emits is equal to the amount of radiation it absorbs. However, the emitted radiation will have a frequency distribution that depends solely on the temperature of the emitting object, so any knowledge of the frequency of the absorbed radiation is lost. In principle, if the blackbody is hot enough, it won't be invisible in the sense of falling outside the visible range of the EM spectrum, and in any case it will certainly be visible to a detector sensitive to the appropriate frequencies.

Also, don't confuse black holes with blackbodies. A black hole has an escape velocity equal to the speed of light at its event horizon. A blackbody is just a perfect absorber of radiation. While all black holes are blackbodies, not all blackbodies are black holes.

[Phishfood]

Also, don't confuse black holes with blackbodies. A black hole has an escape velocity equal to the speed of light at its event horizon. A blackbody is just a perfect absorber of radiation. While all black holes are blackbodies, not all blackbodies are black holes.

Thats what she said.

This whole thing reminds me of a similar but not quite question I asked once in high school. What would happen if you got a perfect one way mirror and made a glass sphere out of it? Pretty much the same result I suppose, but in my head I imagined you could throw it at someone, it would crack and all the light would escape - laser grenade :D

I am now older and wiser. Well, older anyway.

[Brother Oni]

Thanks for all the answers everybody.

However, the emitted radiation will have a frequency distribution that depends solely on the temperature of the emitting object, so any knowledge of the frequency of the absorbed radiation is lost. In principle, if the blackbody is hot enough, it won't be invisible in the sense of falling outside the visible range of the EM spectrum, and in any case it will certainly be visible to a detector sensitive to the appropriate frequencies.

I assume that if this theoretical material absorbed all visible light instead, rather than all incidental EM radiation, it would still have the same visible appearance to people, but would be a temperature based off the ambient temperature/material difference (its heat transfer coefficient?) rather than equal to the amount of radiation it absorbs?

[Douglas]

Thats what she said.

This whole thing reminds me of a similar but not quite question I asked once in high school. What would happen if you got a perfect one way mirror and made a glass sphere out of it? Pretty much the same result I suppose, but in my head I imagined you could throw it at someone, it would crack and all the light would escape - laser grenade :D

I am now older and wiser. Well, older anyway.

Sadly, the real answer to this is that it's a problem strictly for philosophers because perfect one way mirrors are not possible.

[Weezer]

Sadly, the real answer to this is that it's a problem strictly for philosophers because perfect one way mirrors are not possible.

Or 1st/2nd year physics students still living in their ideal frictionless vacuum world.

EDIT: Also assuming this materials perfect absorbtion extends to the frequencies used for radar , it would be the perfect stealth fighter, completely indetectable by radar.

[shawnhcorey]

EDIT: Also assuming this materials perfect absorbtion extends to the frequencies used for radar , it would be the perfect stealth fighter, completely indetectable by radar.

Yeah, but don't forget where you parked it because you never find it again if you don't.

[RebelRogue]

It will look like this:

Spoiler

Show

[The Extinguisher]

Quoting an excerpt for emphasis:


In fact, what would happen with a perfect absorber is that it would change temperature until the amount of radiation it emits is equal to the amount of radiation it absorbs. However, the emitted radiation will have a frequency distribution that depends solely on the temperature of the emitting object, so any knowledge of the frequency of the absorbed radiation is lost. In principle, if the blackbody is hot enough, it won't be invisible in the sense of falling outside the visible range of the EM spectrum, and in any case it will certainly be visible to a detector sensitive to the appropriate frequencies.

Also, don't confuse black holes with blackbodies. A black hole has an escape velocity equal to the speed of light at its event horizon. A blackbody is just a perfect absorber of radiation. While all black holes are blackbodies, not all blackbodies are black holes.

This.

An object that absorbs all light would be emitting light of a certain frequency distribution, so it would appear visible. Hell, a star is a great approximation of an object that absorbs all light, since the reflected radiation is negligible compared to the blackbody radiation.

Also, even if it emitted no visible light (lets say it wasn't hot enough) the brain would do something. The brain is a crazy thing, able to extrapolate and reinterpret optical signals. It's why an apple still appears to be the same colour at noon and at sundown, despite different frequencies of photons being reflected off of it more.

Actually, come to think of it, your brain my just treat it like your blind spot, and if it was small enough, may fill it in with whats in the surrounding area, so it would pretty much be invisible

[Heliomance]

It's actually possible to see this in real life, though it does need to be deliberately set up. Basically, you can set up an opaque container with a hole in the lid, with internal dimensions such that light going in through the hole bounces around inside and is entirely absorbed before any of it manages to get out through the hole again. The hole will then look perfectly black.

[Ted_Stryker]

I assume that if this theoretical material absorbed all visible light instead, rather than all incidental EM radiation, it would still have the same visible appearance to people, but would be a temperature based off the ambient temperature/material difference (its heat transfer coefficient?) rather than equal to the amount of radiation it absorbs?

I'm not sure if I understand your question. If we're talking about a room-temperature object that absorbs all visible radiation, it will appear black to human eyes.

[Sacrieur]

An object that doesn't reflect light is simply black to the eye. But nothing absorbs 100% of all light, it only seems that way because the human eye can't detect small rest of the light that is still reflected.

There is some research to create a "perfect black" which would absorb absolutely every light but until now there is no such thing, even if there are some promising developments in that direction. It would still look like normal black to human eyes.

May I interest you in a black hole?

[Erts]

Black holes are invisible.
(Source: recent National Geographic that I just read.)
So, I think this thing might be invisible, but also leave a shadow because light isn't getting by it.
Does it also absorb other forms of electromagnetic radiation? Such as radio, microwave, infared, ultraviolet, X-rays, and gamma rays? Because we can detect those without our eyes.
The spectrum.